Compositions for dental applications generally consist of methacrylic-type monomers which react “on-command” through a chain-growth, free radical polymerization mechanism. Known dimethacrylate systems in the art are popular for dental fillings and other dental prostheses due to their unique combination of properties, such as fast free radical polymerization, good mechanical properties and an aesthetic appearance. Commonly used acrylic monomers in the dental field are generally linear aliphatic or partially aromatic core groups with a terminal methacrylate functionality, such as 2,2-bis[p-(2′-hydroxy-3′-methacryloxypropoxy)phenyl]-propane (commonly referred as BisGMA) and urethane dimethacrylate (commonly referred as UDMA), as shown in FIG. 1.
In the field of dental materials, carbamate (or urethane) methacrylate derivatives have been widely described and used as monomers for dental applications. For example, UDMA synthesized by reaction of 2,2,4-trimethylhexamethylene diisocyanate with 2-hydroxyethyl methacrylate (HEMA) has found practical applications as a constituent of dental adhesives and dental composites materials. However, the refractive index of UDMA (ηD=1.483) is lower than dental filling materials (ηD=1.52-1.56), so UDMA and others known in the art aliphatic urethanes (for example the ones describe in U.S. Pat. No. 4,813,875, U.S. Pat. No. 4,879,402 and U.S. Pat. No. 7,078,446) have to be used in combination with a high refractive index monomer, like bisphenol-A-diglycidyl methacrylate (Bis-GMA) to provide not just increase in refractive indices but also to increase the mechanical properties of the final dental compositions.
Even though, UDMA in combination with Bis-GMA are present in a wide range of commercial dental restorative materials, the use of such monomers are not without disadvantages. They are generally extremely viscous monomers and are typically diluted with low viscosity methacrylic monomers, such as triethyleneglycoldimethacrylate (TEG DMA), among others (FIG. 2).
Furthermore these monomers and their polymers have several critical deficiencies that limit their clinical performance in dental restorative compositions. For example, existing monomers present relatively low conversion, excessive polymerization shrinkage, poor toughness and excessive water uptake, which are undesirable properties. Known systems often can only reach a final double bond conversion of 55 to 65 percent, which not only contributes to insufficient mechanical properties and wear resistance, but also jeopardizes the biocompatibility of the materials due to leachable, unreacted monomers. As mentioned briefly above, existing dimethacrylate monomers often exhibit significant volumetric shrinkage during polymerization and the induced shrinkage stress results in tooth-composite adhesive failure, initiating microleakage and current caries, which can significantly reduce the longevity and utility of the dental restorative composite. Attempts to increase the final double bond conversion to reduce the unreacted monomers unfortunately lead to an increase in volumetric shrinkage and shrinkage stress.
Several approaches to increase conversion and reduce curing shrinkage have been reported. The amount of shrinkage can be reduced to some extent by increasing filler content. However, when the filler content is too high, it is sometimes difficult to mix the fillers with organic resins. Chemical approaches to increasing conversion and reducing curing shrinkage have been mainly focused on the development of new monomers. One alternative which addresses the shortcomings present in common resins is the use of methacrylate derivatives with high molecular weights. However, typically, the synthesis of these monomers often requires several reaction and purification steps and/or occurs at high temperatures or the synthesized materials are crystalline products which is detrimental for shelf life of the materials due to the risk of precipitation of the solid monomers in the dental compositions with the time. In the other hand, media reports about the possible harmful effects of bisphenol-A (BPA) derivatives have created public concern regarding the presence of Bis-GMA as component in dental materials. Therefore, there is an unmet need in the art for monomers capable of flowing and capable of undergo polymerization that can be produced in a relatively minimal number of steps and which can provide reduced potential for toxicity, reduced polymerization shrinkage, and increased conversion.
The present invention meets the unmet needs of the art by providing viscous liquids which are not bisphenol A-based, and which are high conversion, low volume shrinkage multi-functional carbamate-methacrylate derivatives with refractive indices that match those of dental fillers and have mechanical properties that can be tailored according to the final dental application.